In recent years, a power steering apparatus has been developed wherein the steering force of a driver is mitigated under motor control by detecting the steering force of a vehicle. Attention is currently focused on a brushless motor used in this power steering apparatus. The brushless motor has a rotor equipped with a permanent magnet. It drives the inverter circuit in response to the signal of detecting the rotational position as the position of the magnetic pole of the permanent magnet of the rotor. Thus, the magnetic field for rotation is produced in the stator to provide a highly efficient output. Since it is a brushless motor, it provides a long service life.
The brushless motor is controlled by either a 120-degree drive method characterized by a simpler configuration of the drive apparatus or by a 180-degree drive method for providing a smoother output torque. The 180-degree drive method is commonly employed in the electric power steering apparatus and others involving a problem with pulsation of the motor output torque (hereinafter referred to as “motor torque”).
A sinusoidal wave drive method is commonly used to reduce the vibration and noise of a brushless motor. In this case, the maximum voltage applicable to the motor is √{square root over (3)}/2 (approximately 86.6%) of power voltage Ed. However, the electric power steering apparatus and others is required to provide the assist force (motor drive speed and motor torque) of an electric actuator in order to ensure a quick response to sudden steering wheel operation. This requires a higher applied voltage to be delivered to the brushless motor from the inverter circuit.
To solve this problem, the following technique is commonly known, as disclosed in the Japanese Patent Laid-open No. 2002-345283, for example. The motor is controlled according to sinusoidal wave drive method in the normal mode. When high speed drive and high torque are required (in the torque priority mode), this method is switched over to the trapezoidal wave or rectangular wave drive method.
In this case, if the sinusoidal wave drive is switched over to the rectangular wave drive, a big fluctuation in the applied voltage is caused by the difference in the waveform component at the time of switching, with the result that a fluctuation in the motor output torque occurs. To solve this problem, the amplitude of the modulated wave is increased relative to the PWM modulated wave, as disclosed in the Japanese Patent Laid-open No. Hei 11 (1999)-285288, for example. Then the drive method is switched over to the 1-pulse rectangular wave drive method through the intermediate waveform between the sinusoidal wave and the trapezoidal wave, while the voltage is saturated. At the same time, phase control is provided in such a way as to minimize the torque fluctuation at the time of switching operation. Such a control technique is commonly known.